534 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



of the undulation is not uniform in all segments of the arterial system. 

 It progressively diminishes from the centre to the periphery, and in- 

 creases with the resistance and thickness of the arterial walls. It is, 

 therefore, more rapid in the arteries of the inferior extremities. 



It has been found that in man in the arterial system of the upper 

 extremities the pulse-wave travels with a velocity of 5.8 meters per sec- 

 ond. In the arterial system of the leg the velocity of movement is about 

 6.4 per second. In } r oung individuals before maturity, and when, there- 

 fore, the arteries are more extensible and, consequently, less elastic, the 

 velocity of the pulse-wave is diminished, it then being only about four 

 meters per second. So, also, the causes which reduced the blood-pres- 

 sure will also reduce the rapidity of movement of the pulse-wave. It 

 must not be forgotten that the time of appearance of the pulse-wave in 

 any point of the arterial system by no means indicates that the blood 

 thrown out from the left ventricle would in that interval reach the point 

 at which the pulse-wave is perceived ; for by comparing the velocities 

 of movement of the blood, even in the vessels where the velocity of 

 movement is highest, and the velocity of movement of the pulse-wave, it 

 will be found that the latter moves with many times the higher velocity. 

 The onward current of the blood in the arteries at points at a distance 

 removed from the heart is due to the blood being mechanically pushed 

 forward by the increased quantities thrown into the vascular system by 

 the contraction of the ventricle. 



When the finger is applied over a superficial artery resting upon 

 some firm surface, as on a bone, a series of impulses are felt which coin- 

 cide in number with the contractions of the heart. They are not, how- 

 ever, synchronous with the heart's contraction, but each dilatation of 

 the artery will occur at an appreciable interval after the heart's con- 

 traction, the length of that interval corresponding with the distance 

 of the point examined from the heart. This intermittent expansion is 

 called the pulse, and corresponds to the intermittent outflow of the blood 

 from a severed artery, and is present in the arteries only, being absent, 

 except under certain circumstances, from the capillaries and veins. 



The practical phenomena concerned in the production of the different 

 degrees of the pulse-wave may be reproduced by forcing fluid intermit- 

 tent^ through a tube with elastic walls, in which a variable resistance 

 may be introduced, and by so arranging movable levers in contact with 

 the walls of the tube as to enable them to record their movements on a 

 revolving surface. 



The following diagram, after Marey (Fig. 222), represents the curves produced 

 by a series of levers placed at intervals of twenty centimeters along an elastic 

 tube, into which fluid is forced by the intermittent strokes of a pump. With each 

 stroke of the pump each lever rises and then falls, thus describing a curve and 

 indicating an expansion of the tube, which travels along its walls in the form of 



